The continued cooperation of Najibullah Zazi with the investigation into his attempt to carry out terrorist attacks in New York City has led to the arrest of a fourth suspect, a Pakistani citizen, two days ago, and has shed some new light on the details of the case and the specific targets Zazi and his co-conspirators intended to hit.

Few developments in the case have come out of Pakistan, where the man, who has not been named, was arrested. Authorities say he will be extradited to the United States soon, but that "he may not ultimately be charged in connection with the plot." More details are expected to emerge after investigators have had time to question the Pakistani suspect and determine if any other conspirators are likely to have been involved.

While Zazi was under surveillance by the FBI for months after his trip to Peshawar in Pakistan from August 2008 to January 2009, the federal agency was forced to act after he was pulled over by police in September of 2009 while crossing the George Washington Bridge into New York City. Though police were suspicious, Zazi was free to enter the city, where he stayed overnight in a Queens home before returning to Denver on a Sept. 12 flight. The FBI later overheard a phone call between Zazi’s imam, who also was a police advisor, who warned Zazi he was under suspicion. Federal agents then raided Zazi’s residences and his rental car, and seized evidence linking him to the conspiracy to conduct attacks in the US.

The Queens home where Zazi stayed was subsequently raided, where authorities found bomb-making evidence with Zazi's fingerprints. The evidence was consistent with the bomb-making notes retrieved by investigators on Zazi’s laptop. Zazi had also purchased unusually high quantities of hydrogen peroxide and products containing acetone in Denver-area beauty supply stores. These chemicals are used to produce an explosive known as TATP.

In the following weeks and months, authorities were able to build a strong case against Zazi and his co-conspirators Adis Medunajin and Zarein Ahmedzay. The three men were eventually charged with conspiring to use weapons of mass destruction and supporting al Qaeda.

In a February court hearing, Zazi listened to a judge explain all the charges against him after waiving his right to a grand jury hearing. Addressing each count, Zazi responded "guilty" on every occasion. These guilty pleas gave prosecutors the tools they needed to gain Zazi's cooperation. In the ensuing questioning and court hearings, Zazi revealed his meticulously thought-out plot.

Zazi and his co-plotters planned to separate out on Sept. 14, 2009, at New York's two busiest transit stations, Grand Central Station and Times Square, with explosives strapped to their bodies. There they would board the numbers 1,2,3,and 6 lines and find their way to the middle of the train cars. During peak rush hour the three men would detonate their explosives to inflict the highest possible number of casualties.

In the initial questioning and interrogation sessions, Zazi had confessed to authorities that he and others had traveled to Pakistan and received terrorist training. According to Customs and Borders Protection records, Zazi and "others" boarded Flight Number 84 on Qatar Airlines on Aug. 28, 2008. Departing from Newark, New Jersey, they landed in Peshawar. After three-and-a-half months of training with al Qaeda in the Waziristan region, Zazi returned to the US to execute his plot. On Jan.15, 2009, Zazi boarded Qatar Airlines Flight Number 83 to JFK Airport in Queens, New York.

Court documents obtained by CBS reveal Zazi's activities in Pakistan and portray the matter-of-fact manner in which Zazi spoke of his plot and the seriousness of his intentions in carrying it out.

When the court asked Zazi what his intentions were in to traveling to Pakistan, he stated: "Our plan was to go to Afghanistan to fight with the Taliban. While we were in Peshawar, we were recruited by al Qaeda, instead. We were taken by al Qaeda to a training camp in Waziristan, where we received weapons training."

Although not explicitly stated, Zazi was recruited by al Qaeda’s external operations branch, the division assigned to carry out attacks against the West and inside the US.

Zazi then elaborated on how his intended target switched from US troops abroad to US civilians at home. "During the training, al Qaeda leaders asked us to return to the United States and conduct a martyrdom operation. We agreed to this plan."

During training, it seems that Zazi impressed leaders at the training camps with his potential in becoming a specialist in explosives, eventually being designated leader of the small terrorist cell. "Later, I received more training from al Qaeda about how to construct explosives for attack in the United States. During my training, I had discussions with al Qaeda leaders, including target locations, such as New York City subways." In further discussing the plot, the judge stopped Zazi to clarify his statement: “You used the word 'bomb.' Do you mean bomb or bombs?” “Bombs,” said Zazi. “Plural?” the judge asked. “Plural,” Zazi responded.

The plot concocted by Zazi and his co-conspirators (who, unlike Zazi, have pled not guilty), bears resemblance to the 2004 Madrid train bombings carried out by an al Qaeda-aligned cell. In that attack, multiple coordinated backpack bombs were detonated to ensure maximum casualties, ending in 191 deaths. The New York plot resembles even more closely the 2005 attacks on the London transit system, which killed 56 and wounded hundreds more when four suicide bombers detonated their explosives during rush hour.

These latest disclosures regarding al Qaeda's attempted reach come during a time of mounting concern and heightened security in light of the recent deadly Moscow transit bombings. Just two weeks ago, two female Black Widow suicide bombers detonated their explosives at separate metro stations, killing 39 people. The al Qaeda-affiliated Caucusus Emirate eventually took credit for the attacks.